The present invention relates to ignition control systems of the type used to ignite a gaseous fuel; and more particularly, it relates to an ignition control system which ignites the fuel directly, as distinguished from the type which first ignites a pilot flame which, in turn, ignites the main burner. A system of this type is referred to as a direct ignition system in this art.
Direct ignition systems are known, but for the most part they employ sources of alternating current to supply the electrical energy rather than direct current (dc). Such systems normally have a predetermined period after the main fuel valve is opened during which ignition is attempted. If ignition is successful, a flame is established, and a flame probe (which may be a pair of electrodes placed in the flame and spaced apart) is used to detect the presence of a flame. If a flame is present and an ac voltage is applied to the electrodes, the ionization of gas molecules caused by the presence of the flame will permit current to flow unidirectionally between the electrodes. Thus, the flame probe acts as a rectifier if an ac voltage is applied if and only if a flame is present.
A problem arises where it is desired to use such a system in an environment where the only source of electrical energy available is a battery or other dc source, such as in a recreational vehicle, camper or the like, in which the output of the alternator resembles a full-wave-rectified voltage. If the flame sensor relies on bidirectional conductance rather than rectification, a piece of fallen scale bridging the electrodes may create a false signal indicating the presence of a flame when none is in fact present.
In the past, it has been the practice to use an inverter to convert the dc power to ac power and use a system of the type described wherein the presence of a flame is detected by means of the rectification phenomenon mentioned above. An inverter obviously adds cost to such a system and it is also wasteful of electrical energy in use.
Another problem arises in systems of this type--namely, when a flame-out or interruption of fuel occurs each manufacturer of a different furnace (or other gas-fired appliance) may recommend a different delay period before a new ignition cycle is attempted. That is, depending upon the design of the appliance, it may be desirable to allow time for purging of any remnant fuel from the firebox area. This period is called the re-cycle period, and a manufacturer of ignition control systems must accommodate the variations in re-cycle time likely to be encountered.
Another area of design in fuel ignition systems in which improvement is continuously sought is that of safety. Not only is it desired that lockout occur if the operating sequence and timing are not followed implicitly, but it is also desirable that the system not malfunction due to component failure, such as the shorting of a capacitor or the like. It is thus a principal object of the present invention to provide an ignition control system which may be used directly with a source of dc electrical power (or rectified ac power) without the need of an inverter, and to provide such a system with the desired interlocks for timing and sequence of operation which are a necessary part of a fail-safe system. At the same time, it is desired to provide such a system in which adjustments in the trial-for-ignition period and the re-cycle period are easily accommodated with changes only in circuit component values, and without the need to re-design the circuit functions.
The present invention employs a first relay (called a "flame sensing" or simply "flame" relay) and a checking relay. The checking relay is connected in circuit with a thermostat and a manually actuated switch called a "remote" switch, which may be used to re-cycle the ignition control system in the event of a flame-out or shut down for loss of fuel.
The two relays are interlocked by means of their contacts to insure that the system operates in proper sequence before the main fuel valve is actuated. A trial-for-ignition circuit is arranged to energize the coil of the flame relay, and it includes a timing circuit and a drive circuit. In response to a call-for-heat signal from the thermostat, the checking relay opens a set of contacts interposed between a source of dc power and the timing and drive circuits. At the same time, a capacitor in the timing circuit is connected to energize the drive circuit which, in turn, energizes the flame relay for a predetermined trial-for-ignition time. Energy is supplied to the drive circuit and the flame relay by an initiation timing circuit which also includes a charged capacitor isolated from the power source temporarily during an ignition cycle.
A flame probe, adapted to sense the presence of a flame at the burner, is connected in circuit with the timing circuit when the checking relay is energized, and if the presence of a flame causes the probe to conduct bilaterally, the timing capacitor is prevented from completely discharging. Hence, the drive circuit for the checking relay remains actuated. However, should flame be lost for any reason, the timing circuit will commence a re-ignition time-out cycle (which is slightly shorter than the original period), and if a flame is not reestablished during this period, lockout will occur. Further, if a piece of conductive material becomes lodged between the flame electrodes so as to cause a false conduction signal, when the call-for-heat is satisfied and the ignition circuit is de-energized when the thermostat contacts open, the control system cannot be re-started because of the interlock with the flame sensing relay, which remains energized.
If a lockout does occur through a loss of flame for greater than a predetermined time, the system can be re-started by cycling either the thermostat or the remote switch. In this case, when either the thermostat or remote switch is opened, a charge limiting resistor is connected in circuit with the timing capacitor to limit the charging time for that capacitor. The capacitor must charge to a predetermined voltage before it can store sufficient charge to actuate the drive circuit. Hence, the re-cycle time can be adjusted by varying the charge-limiting resistor. Further, the trial-for-ignition time can be varied by varying the value of the capacitor in the timing circuit.
When the flame relay is energized during the trial-for-ignition period, it energizes the checking relay through the thermostat contacts. If both relays are operated in the proper sequence, they cooperate to energize the main fuel valve as well as a spark generator circuit. The spark generator circuit is an oscillator, the output of which is coupled to a spark gap for ignition fuel from the main valve. An enable circuit is connected to the spark gap to enable the spark generator if the spark gap is non-conducting (i.e. a flame is not present). During ignition sparks strike across the spark gap with a random polarity, and eventually a charge of predetermined polarity appears on a capacitor connected across the spark gap to disable the enable circuit, thereby disabling the spark generator. The spark generator remains disabled for so long as the spark gap remains in its relatively high conductance state (indicating the presence of a flame).
With the present invention, if a piece of scale drops across the spark gap, the spark generator will be disabled. If this occurs prior to ignition, the flame probe will cause the timing circuit to time out and lock the system out since no flame is sensed. If it occurs during a fuel cycle, after the cycle terminates or a flame-out occurs, the system will lock out for the same reason.
If a piece of scale drops across the flame probe while the system is operating to supply fuel, operation continues normally, but the system will not re-start. If, on the other hand, a piece of scale shorts the flame probe electrodes when the system is not energizing the main fuel valve, then the system cannot commence an ignition cycle.
Thus, the present invention provides a fail-safe ignition control system for a gas-fired appliance which can use dc power directly without the need for an inverter.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing.